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Autumn Sky 2015 & Lunar Eclipse

Topic: Moon
Article Published - September 23
by Dr. Jim Lattis, Director, UW Space Place
September Night Sky

For the first time in many months, our evening sky is devoid of bright planets. Saturn sets soon after the Sun no others rise until the predawn hours. An early, planetless autumn is the perfect time to pay attention to some important geometrical figures that help us learn our way through the constellations. Depending on when you choose to observe, these can be very high overhead, so simply lying horizontally on your back, looking up, is the least fatiguing way to look for them.

By now the Summer Triangle, the geometrical figure defined by three of the brightest stars in the sky, has been a prominent feature of the evening sky for months, and it will remain visible for a while yet. If you are lucky enough to be under a truly dark sky, you'll see the summer Milky Way arcing through the Triangle.

Just to the east of the Triangle is the Square, usually called the Great Square of Pegasus. The stars of the Square are not so prominent as those of the Triangle, and it isn't really a perfect square, so it can take some patience to identify it the first time or two. While the Triangle spans and encloses several constellations, the Square touches only two: Pegasus itself and Andromeda. The star at the northeastern corner of the Square actually belongs to Andromeda, whose stars curve off from there and point roughly at Perseus. The stars of Perseus are, roughly speaking, two arcs of stars that connect at the top.

North of the Square look for Cassiopeia, whose stars make a slightly distorted letter "W". Look again for the Milky Way and notice that it stretches from the northeastern horizon, past Perseus, through Cassiopeia, through the Summer Triangle, and down to the horizon again.

Also in the sky this fall is a particularly nice total lunar eclipse, which, if the weather cooperates, will be a fine site in Wisconsin. Lunar eclipses always happen at full moon, because that is the only time it is possible for the Moon to pass through Earth's shadow. As the full moon closest to the autumnal equinox, which falls this year on 23 September, this will be 2015's Harvest Moon. So it's an eclipse of the Harvest Moon. Because the eclipse happens to occur very close to the time of perigee (the point in the Moon's orbit when it comes closest to Earth), it will be near its greatest apparent size from our point of view. Though the term is misleading, this is often called a "super moon." In truth, its closest approach is not that much closer than its average distance, so the "super moon" effect is largely in the eye of the beholder.

Because month's is a relatively deep eclipse, totality lasts nearly an hour and a quarter. In North America's midwest, the Moon will be well up in the eastern sky (moonrise occurs about a half hour before the eclipse begins) and high overhead as the event ends around midnight. Specific times for the import stages of the eclipse are listed below and shown graphically in (this diagram)

Time (CDT)
19:10 Penumbral eclipse begins (Moon contacts penumbra)
20:07 Partial eclipse begins (Moon contacts umbra; start of the interesting part)
20:55 Moon at perigee (hence "super" moon)
21:10 Total eclipse begins (Moon immersed in umbra)
21:47 Middle of eclipse
22:24 Total eclipse ends (Moon begins to emerge from umbra)
23:27 Partial eclipse ends (Moon exits umbra; end of the interesting part)
00:24 Penumbral eclipse ends (Moon exits penumbra)

Observing a lunar eclipse is about as easy as astronomical observing gets. You don't need a finding chart to locate the Moon! And unlike a solar eclipse, where eye damage is a distinct hazard, observing a lunar eclipse requires no precautions or special equipment. Just find a comfortable place under a clear sky. Binoculars or a small telescope aimed at the Moon can add a lot to the experience. It all happens at a slow, majestic pace, so there's plenty of time to compare naked-eye with binocular views and to come and go, even relocate, during the event.

From the University of Wisconsin's "Space Place"
Villager Mall, 2300 S Park Street, Madison

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